Why does water spiral down a drain?

A popular myth says the Coriolis effect makes water spin one way in the Northern Hemisphere and the other way in the Southern. In reality, the Coriolis force is caused by Earth’s rotation and does influence large systems like hurricanes and ocean currents. But for a small basin, the force is millions of times too weak to overcome other factors. The direction of the spiral is set by tiny residual currents in the water, the shape of the basin, how the water was poured in and asymmetries in the drain. In carefully controlled laboratory conditions with perfectly still water, the Coriolis effect has been detected, but in everyday life it is overwhelmed.